Power switches, such as insulated gate bipolar transistors (IGBTs) can be used to control electric power transfer to an appliance such as a motor. Often such switching is used to effect pulse width modulation (PWM) to control frequency and/or current transferred to the appliance.
Stray inductance in emitter and collector terminal connections of such-switches are often relatively large due to the mechanical design. This typically leads to overshoots of the voltage across the switch.
One solution to this problem is to select switches which have ratings that are sufficient to withstand the overvoltages resulting from such overshoots. However, such over-dimensioning of components is unnecessarily expensive and may require more physical volume.
Other solutions include limiting the switching speed or times, and thereby the overshoots, e.g. by using a relatively large gate resistor in series with a voltage stiff driver stage buffer. The gate charge current of the switch is then limited. The drawback with this solution is that if low overshoots in voltage and current (the current overshoot being a result of diode recovery current) shall be accomplished the switching times need to be slowed down considerably more than optimum.
US2002-0070772 presents an active resistance controlled to modify a drive signal provided to a gated device such as an insulated gate bipolar transistor (IGBT). The active resistance is between an input lead that receives an input drive signal, such as from a conventional gate driver IC, and an output lead at which an output drive signal is provided to the device's gate. The active resistance is controlled in response to a feedback signal that includes information about the output drive signal, so that the output drive signal is a modified version of the input drive signal. To reduce di/dt and hence control EMI emission, the output drive signal can include turn-on and turn-off transitions where the input drive signal includes steps. Such a solution is complicated and can be liable to ill-effects if controlled incorrectly.
Hence, there is a need for a robust solution of how to reduce voltage overshoots when driving power switches.